420 APPENDIX II. 



wire for instance a common knitting needle in such a form 

 as to hold the card upright between its ends and to slide on 

 the rod. This is the whole apparatus. It is to be used by 

 candle-light in this manner : Attach to the fixed pins a 

 small lock of the wool to be examined, containing 20 or 30 

 single fibres, and look through them and through the 

 central hole at a candle not very remote from the card, 

 the blackened side of the card being turned towards the 

 eye. The hole will appear to be surrounded by a bright 

 surface, reddish at the margin, by a dark circle or ring, and 

 again by a brighter ring, bluish-green within and red 

 without. If the colours are not seen distinctly, we may 

 conclude that the wool is mixed and not perfectly fit for 

 examination ; but in this case we may generally form an 

 estimate of its quality by means of the bright surface only, 

 moving the card along the rod until the holes of the inner 

 circle appear exactly at the extreme margin of that surface, 

 where it is of a reddish hue, the place of the card as 

 indicated by the scale showing the number which cha- 

 racterises the wool. It will also be most convenient to 

 begin by producing this coincidence of the inner circle in 

 other cases where the outer ring of colours is more 

 distinctly seen, and then to adjust the card more accurately ; 

 so that the holes in the circumference of the outer circle 

 may appear to coincide with the middle of the coloured 

 ring at the common limit of the red and greenish portions. 

 If the holes be seen in the red ring, the card must be 

 brought nearer to the edge; if in the blue, it must be 

 moved farther off, and the number of the scale must be 

 observed as before. It will require very attentive ob- 

 servation, and perhaps some practice, to obtain always pre- 

 cisely the same number for the same substance, but by 

 taking the mean of several trials we may be perfectly 

 certain of coming within a unit of the true place of the 

 slider, and perhaps even of avoiding any error at all. 



I have not yet had an opportunity of examining any 



